This invention relates to coding for multi-user communication.
We use the following acronyms in our discussion:
As shown in FIG. 2, in wireless cellular communications, a large geographic area is divided into cells, each served by a single base station 10 communicating with multiple mobile stations 12 in its cell.
3GPP2, a standards organization developing third generation wireless communication standards, has developed a new wireless air interface standard called 1xEV-DO (3GPP2 C.P9010, CDMA2000 High Rate Packet Data Air Interface Specification, incorporated by reference; see also, A. J. Viterbi, CDMA Principles of Spread Spectrum Communication). This standard supports high-speed data communications between mobile stations (MS""s) and their base station (BS) at bit rates of up to 2.457 Mbit/s in the forward link (from BS to MS""s) using only 1.25 MHz of spectrum. However, 1xEV-DO does not efficiently support delay-sensitive, low-rate traffic, such as voice.
3GPP2 is developing a new air interface standard called 1xEV-DV, which supports three modes of traffic delivery: (1) Real-time, (2) Non-real-time, and (3) Mixed real-time/non-real-time. This standard is intended to be inter-operable with the CDMA2000 1x standard.
Lucent, Inc. has made a proposal to 3GPP2 for the 1xEV-DV standard (3GPP2 C50-2000-0918-014, xe2x80x9c1xEV-DV Forward Channel Structure,xe2x80x9d Lucent Technologies, September 2000; and 3GPP2 C50-2000-0918-015, xe2x80x9c1xEV-DV Proposal,xe2x80x9d Lucent Technologies, September 2000, both incorporated by reference) that includes two modes: single antenna and multi-antenna (called MIMO). We discuss only the single antenna case here although our invention can also be applied to MIMO.
Lucent""s 1xEV-DV proposal defines new channels on top of channels used in the CDMA2000 1x standard. One of the new channels in the forward link is called the Forward Packet Data Channel (FPDCH).
The FPDCH uses Walsh Codes that are not used by other channels, such as fundamental channels that are used for voice. Multiple MSs share the FPDCH as a common, dynamically changing code space in a time multiplexed fashion. Each MS continually predicts the SIR from all active BS""s using the burst pilot and/or the continuous pilot. From the SIR information along with the allocated power and code space for the FPDCH, the MS can choose a forward link rate and give notice of this rate to the BS through the reverse link RAI channel.
The MS also predicts whether multiple transmit antennas would aid the communication for its channel conditions. The FPDCH is shared by packet data MSs using time-multiplexing.
As shown in FIG. 3, the FPDCH 16 includes the following time-multiplexed subchannels: the Forward Packet Data Pilot Subchannel 22, the Forward Packet Data Traffic Subchannel 24, and the Forward Packet Data Preamble Subchannel 26. The Traffic Subchannel carries customer data packets.
Each subchannel is further decomposed into code division multiplexed quadrature Walsh channels. The number of Walsh channels may vary in time. Each of the parallel streams of the FPDCH is covered by a distinct 16-ary/32-ary/64-ary/128-ary Walsh function at a chip rate that will yield Walsh symbols at 76.8 ksps/38.4 ksps/19.2 ksps/9.6 ksps. The Walsh-coded symbols of all the streams are summed together to form a single in-phase stream and a single quadrature stream at chip rates of 1.2288 Mcps. The resulting chips are time-division multiplexed with the preamble and pilot subchannel chips to form the resultant sequence of chips for the quadrature spreading operation.
The preamble is a QPSK-modulated channel that has all xe2x80x980xe2x80x99 symbols sent on the in-phase channel. The in-phase signal is covered by a 32-chip bi-orthogonal sequence and the sequence is repeated several times depending on the RAI rate. The bi-orthogonal sequence is specified in terms of the 32-ary Walsh functions and their bit-by-bit complements which represents the 6-bit MAC address of the destination.
The Forward Packet Data Traffic Subchannel encoder packets can be transmitted in 1 to 32 slots as shown in Table 1, below, where one slot is 1.25 msec. For each RAI rate, the encoder packets are divided into 1, 2 or 4 sub-packets. Each subpacket can have 1, 2, 4, or 8 slots. Each slot consists of 320 chips of pilots and 1216 chips for data and preamble.
The Lucent proposal uses Asynchronous Incremental Redundancy, which works as follows. When an MS is scheduled to be serviced on the FPDCH, rather than sending the entire encoder packet to the MS at an effective rate that is equal to the RAI rate, the BS initially transmits only the first sub-packet. In this case, in the preamble, the Sub-Packet Sequence Number is set to xe2x80x9800xe2x80x99. The sub-packet is preceded by the preamble whose length is dictated by the RAI rate.
The actual length of preamble is given in Table 2, below.
If the MS detects the preamble and succeeds in decoding the sub-packet, transmission is successful and the MS sends an ACK back. The effective rate of the MS in this case is equal to the number of sub-packets times the RAI rate.
If the MS detects the preamble but is not successful in decoding the sub-packet, it sends a NACK back. In this case, the next sub-packet is transmitted by the BS the next time the MS is scheduled for service. Then, the Sub-Packet Sequence Number is set to xe2x80x9801xe2x80x99.
Due to the asynchronous nature of the IR operation, every sub-packet needs to be preceded by a preamble. The preamble gives the mobile station information about the sequence number of the sub-packet. During the course of the incremental redundancy operation, each sub-packet may be received with a different preamble size because this value is completely dictated by the current RAI rate.
In incremental redundancy operation, if the user is not able to detect/decode the preamble, it sends neither an ACK nor a NACK. The set of rates signaled by the RAI channel, their corresponding slot and sub-slot structures as well as the encoder packet coding and modulation types are listed in Table 1 for Non-MIMO configurations. Table 1 shows the number of slots and sub-packets required as proposed by Lucent.
The overall Forward Packet Data Channel structure is shown in FIG. 4 for BSs with a single transmit antenna.
In general, in one aspect, the invention features generating packets for delivery over a forward communication channel from a base station to a remote station, in which a parameter that characterizes the packets being generated is determined adaptively based on factors not limited to the data rate of the channel.
Implementations of the invention may include one or more of the following features. The factors may include the levels of power used to send the packets (which will affect rate per dimension at the MS), or the availability of code space used to send the packets, or both. The adaptively determined parameter that characterizes the packets may be the time-length of the packets or the number of time slots, or the number of bits per encoder packet, or forward error correcting scheme, or modulation scheme, or any combination of them. The forward communication channel may be a forward packet data preamble subchannel of a wireless communication system. The preamble subchannel may carry auxiliary information at the same time that MAC address is also being carried. The MAC address part of the preamble data may be coded using bi-orthogonal signaling or other block coding schemes, such as Hamming, extended Hamming, Golay, Reed-Muller, BCH, Reed-Solomon, or parity check codes. The coded address is then spread in one or multiple blocks of Walsh function depending on the number of dimensions or code space available for the preamble. The energy required to send the preamble may be spread substantially evenly among the blocks. The remote station may be a mobile wireless station.
Other advantages and features will become apparent from the following description and from the claims.